Plasma arc cutting is a process in which an electric arc is used to cut a metallic workpiece. Generally, plasma arc cutting uses an electric arc between an electrode and the metal to be cut. The arc creates a plasma that cuts the metallic workpiece.
It is generally accepted that 180 volts or higher open circuit is desirable to initiate a plasma arc cutting process. After the process has been initiated, the cutting arc voltage is approximately 90-125 volts.
A typical prior art plasma arc cutting power supply receives an input voltage (from a power line, generator, or other power source) and provides an output voltage to a pair of output terminals, one of which is the electrode and the other of which is connected to the workpiece or a pilot circuit. There are numerous types of known plasma arc cutting power supplies, such as magnetic (generator/alternator) power supplies, inverter power supplies, phase control power supplies, and choppers or secondary switchers. One known power supply is a chopper based power supply such as that shown in U.S. patent application Ser. No. 08/587,901, filed Jan. 16, 1996, entitled Plasma Cutting Or Arc Welding Power Supply With Phase Staggered Secondary Switchers, which is owned by the owner of this invention, and is hereby incorporated by reference.
Often, a plasma cutter is used for applications, and at locations, where electric arc welding is performed. Plasma cutting and welding have some similarities in that both receive an input power and transform or convert the input power into a usable voltage and current for the particular applications. However, plasma cutting is typically performed at a higher voltage then welding, thus an output power suitable for welding is generally not suitable for plasma cutting. At sites were line power is available it is relatively simple to connect the plasma cutter to one power outlet and the welding power supply to another power outlet.
Thus, at sites where an engine/generator is used it is more problematic to provide power for both the plasma cutter and welder. One solution is, of course, to provide a first engine/generator for plasma cutting and a second engine/generator for welding. However, this is relatively expensive in that two engines/generators must be provided.
One prior art attempt to avoid the need for two engine/generators is a plasma cutting power source called StarCut.TM., manufactured by PowCon, which converts a welding power output into plasma cutting power. (See e.g. U.S. Pat. No. 5,086,205.) However, this type of plasma cutter may be used only when welding is not being used, since it derives its power from the same output that is used for welding. Also, a boost convertor is needed to boost the voltage from a welding output to a plasma cutting output, and a dc input power source is needed.
One possible solution which can avoid the added expense of a second engine/generator is to use the auxiliary output of a welding power supply to power a plasma cutter. The auxiliary power of an engine drive power source typically ranges from 3,000 to 10,000 watts of electrical power. This power is used to run a wide variety of auxiliary equipment such as power tools and, possibly, plasma cutters, and is provided with a typical utility line receptacle or connector.
However, plasma cutters need to provide a floating output (i.e. not referenced to ground), and thus must be isolated from grounded power sources. A floating output is needed because it is relatively easy for the user to touch either output, and thus complete a current path to ground if the output is referenced to ground. This problem is particularly serious in plasma cutting, which is performed at a relatively high output voltage, 90 V e.g. (Other areas, such as welding, typically provide a lower output voltage, which is not as dangerous.) Also, the components in plasma cutters are designed to receive a floating input, and a grounded input could cause component failure or malfunction. As used herein, floating output or floating ground means the ground is not referenced to an external, fixed potential.
Typically, a transformer is used to safely isolate the output from the input. (See, e.g. U.S. Pat. Nos. 4,943,669 and 4,918,285.) The use of transformers results in a heavy power source. Alternatively, an inverter may be used, but such power sources are relatively complex and expensive. Thus, using such a prior art plasma cutter on the auxiliary power of an engine drive power source results in either a heavy, non-portable cutter or an expensive portable cutter.
The aux power on an engine/generator is typically a 115 or 230 VAC output (or other utility line voltage). The 230 VAC is obtained from the two 115 VAC outputs. There may be two secondary windings, having a common center tap which is tied to ground. The outer taps provide the 115 VAC or 230 VAC power, which is useful for tools, lights, etc. Thus, the aux power is not typically an isolated (from ground) source of AC power.
For example, a prior art auxiliary output from an engine/generator (such as that used to provide welding power) is shown FIG. 1. The transformer P1 includes a primary winding T1, and auxiliary windings S1 and S2. Transformer T1 (which may alternatively be a generator) typically includes another secondary winding (not shown) used for the welding output. There are two auxiliary outputs, each being 115 VAC. Connecting these outputs in series would provide a 230 VAC output, which is a desirable input for a plasma cutter. However, as shown in FIG. 1, the center leg is connected to ground so that both auxiliary 115 VAC outputs are grounded. Thus, the input to the plasma cutter would not float, as desired, and both outputs would be "hot" relative to ground. Touching either output would put the user at risk of shock. Even if the auxiliary power were isolated, it would be redundant (and expensive or heavy) to run a typical plasma cutter which also has primary isolation.
Accordingly, a plasma cutter capable of being powered by the auxiliary power output of an engine/generator, such as that used to power a welding power supply, is desired. Preferably, such a plasma cutter will receive about 230 VAC input and will provide a plasma cutting output. Additionally, such a plasma cutter will preferably include a ground fault interrupt and/or other protection circuits, to protect the user in case of improper connection, or ground failure.